Add Lint/NoReturnInMemoization cop

[rafaelfranca]

Cop based on https://docs.rubocop.org/rubocop/cops_lint.html#lintnoreturninbeginendblocks, but with the difference that it only cares about instance variables, given sometimes using return inside begin..end blocks is idiomatic Ruby, like when handling exceptions.

Example of such case:

def some_method
  some_value = begin
    get_external_api
  rescue ApiError
    return { error: "Api returned an error" }
  end

  do_something(some_value)
end

This cop checks for the use of return with a value in begin..end blocks in the context of instance variable assignment such as memoization. Using return with a value in these blocks can lead to unexpected behavior as the return will exit the method and not set the value of the instance variable.

# bad
def foo
  @foo ||= begin
    return 1 if bar
    2
  end
end

# bad
def foo
  @foo = begin
    return 1 if bar
    2
  end
end

# bad
def foo
  @foo += begin
    return 1 if bar
    2
  end
end

# bad - using return in rescue blocks still exits the method
def foo
  @foo ||= begin
    bar
  rescue
    return 2 if baz
  end
end

# good
def foo
  @foo ||= begin
    if bar
      1
    else
      2
    end
  end
end

# good - not an assignment to an instance variable
def foo
  foo = begin
    return 1 if bar
    2
  end
end

# good - proper exception handling without return
def foo
  @foo ||= begin
    bar
  rescue
    baz ? 2 : 3
  end
end

[amomchilov]

Nice.